RIO DE JANEIRO (AP) — Japan's shocking win over New Zealand and a tournament-ending injury to Sonny Bill Williams proved on the first day of Olympic men's rugby in 92 years what coaches have long been saying about the sevens format.

"First time Japan has ever beaten our side, but that's sevens rugby now," New Zealand coach Gordon Tiejens said. "The gap has closed considerably. If you're not up for it on the day, you'll get beaten by any team."

World series champion Fiji, second-seeded South Africa and Britain secured quarterfinal spots and topped their pools, but nothing came easily Tuesday at Deodoro Stadium.

Japan's 14-12 upset against New Zealand, its first ever in rugby sevens, and its narrow 21-19 loss to Britain turned the formbook in Pool C upside down.

If that wasn't enough, the Japanese went within inches of a draw with Britain, missing a chance at a late equalizer when Teruya Goto couldn't convert Lomano Lemeki's last-minute try.

"It was heartbreaking. We wanted that goal, so for the next game we'd know we're going to quarterfinals," Japan's Lote Tuqiri said. "Now we need to step up — it's a must-win game for us against Kenya."

Still, he reckoned the performances at the Olympics were worth celebrating almost as much as Japan's win over South Africa last year in the traditional 15-a-side game, which ranked as the biggest upset ever at the Rugby World Cup.

"How Japan finished at the World Cup was really a big boost for Japan rugby and really lifted rugby in Japan," he said. "Hopefully our win against New Zealand will boost it another level."

New Zealand regained some confidence after its bad start with a 28-5 win over Kenya, which opened with a 31-7 loss to Britain.

Tietjens said his team would miss Williams, a two-time World Cup champion with the All Blacks and the highest profile player in the tournament.

"For that to happen in the first game of the tournament is devastating. He's obviously shattered and disappointed," Tietjens said. But "there's a lot of character in this team, I can assure you."

Fiji kicked off its bid for a first Olympic medal with a 40-12 win over host Brazil, then needed two late tries for a comeback 21-14 win over Argentina — ensured by Jerry Tuwai's desperate try-saving tackle on Axel Muller in the last seconds.

The Fijians lead Pool A and the second-seeded South Africans posted 50 points without conceding any in wins over Spain and France to top Pool B.

The top eight teams after three group games enter the quarterfinals. That's the top two teams in each of the three groups, plus the next two third-place teams, so points differentials will be crucial.

The U.S. men conceded a last-minute try in a 17-14 loss to Argentina in their first match and rebounded with a 26-0 win over Brazil, leaving plenty on the line Wednesday when the Americans play Fiji.

In Pool B, the battle for second is between France and Australia. The French had a surprising 31-14 win over the Australians in the opening game of the tournament and then lost 26-0 to South Africa. Australia recovered from its opening loss to beat Spain 26-12, but faces South Africa next.